The most important aspect as regards technique concerned with producing a thin film made of thermoplastic resin is the way in which the shapeless thin film, without substantially containing a so-called crystallized part, is superior in its drawing properties. Therefore, the film is required to be cooled at as fast a rate as possible. That is, the film is required to be rapidly cooled.
Several indirect cooling methods have been proposed wherein melted thin film is contacted with a mandrel for cooling in which the mandrel is cooled by coolant therewithin. However, the rate of cooling by means of an indirect cooling method is lower than that by means of a direct cooling method wherein the melted thin film is directly contacted by the coolant. Therefore, several kinds of apparatus for cooling by directly contacting the melted thin film extruded in the producing step for producing a tubular thin film made of thermoplastic resin have been provided.
For example, there is known a method wherein the melted thin film is solidified by rapid cooling by pressing the melted thin film against a sidewall located outside of tubes in which coolant flows, the coolant being caused to overflow the tubes and flow downward along the sidewall as the melted thin film is pressed on the sidewall. In such device, the coolant is provisionally stored in the lower bubble formed in the underside of a cooling apparatus, and the coolant is discharged by a siphon or a pump (Japanese Examined Patent Publication No. 35192/1970). In another method wherein spirally formed grooves located under a double tube for setting the diameter of the film, the coolant, other than the overflow coolant, flows within grooves in the double tube whereby high speed film productivity is improved. In this device, the coolant is provisionally stored in the bubble formed in the underside of the cooling apparatus, the coolant being discharged by a siphon or a pump, and the like, (Japanese Examined Patent Publication No. 31473/1971). And in still another method, the melted thin film is solidified by contacting the film with the coolant, and the liquid remaining on the film is removed by suction of the liquid (Japanese Examined Patent Publication No. 2072/1964), or like method.
With respect to the method of cooling by means of overflown coolant in accordance with the above-mentioned Japanese Examined Patent Publication No. 35192/1970 and Japanese Examined Patent Publication No. 2072/1964, the overflown coolant flowing downward has a low current velocity. No matter how fast it may flow, the velocity thereof is the same as that of the thin film moving downward. Accordingly, it is difficult to obtain a uniform thin film without having a shape defect since the coolant flowing downward with the thin film becomes high in temperature due to the remaining heat of the thin film so that the cooling capacity of the apparatus is lowered because of partial boiling of the coolant, or there arises a partial bonding caused by contacting directly melted thin film with the mandrel due to an irregularity in the quantity of flow.
Accordingly, in order to produce a thin film with high speed, it is considered to conduct the process by keeping the temperature of the coolant low. However, there is a commercial limitation in such a method. Therefore, it has not been possible to produce the thin film with high speed.
Further, in such direct cooling method, the coolant is directly contacted with the thin film. Therefore, the thin film is wetted so that it is necessary to remove the coolant from the film. However, it is difficult to remove the coolant completely. Accordingly, there are defects due to irregular heating caused by drips or liquid membrane remaining on the thin film when the coolant flowing in the apparatus remains on the film, whereby an irregularity of stretching readily occurs.
Further, with respect to the cooling method disclosed in above-mentioned Japanese Examined Patent Publication No. 31473/1971, cooling is promoted by the lower part of the internal coolant flowing in a spiral groove. However, in the apparatus, the upper side double tube for setting the diameter of the film is cooled by an indirect cooling method. Therefore, there is a limitation of cooling capacity.
Further, with respect to the cooling methods disclosed in Japanese Examined Patent Publication Nos. 35192/1970 and 31473/1971, the coolant is provisionally stored in the lower bubble formed under the cooling apparatus, the coolant being discharged by a siphon or a pump, and the like. In such a method, the stress imposed on the thin film is easily varied by the gravity of the stored coolant. In the extreme case, there is a possibility that the lower bubble is broken. Further, the coolant is not completely removed in the nip rolls portion of the apparatus which collapse the lower bubble. Therefore, the drips of liquid or liquid film remain on the thin film. Consequently, irregular heating occurs when the obtained thin film is stretched. As a result, the irregular stretching easily occurs.
Further, with respect to the cooling method disclosed in Japanese Examined Patent Publication No. 2072/1964 wherein the liquid remaining on the thin film is removed by suction of the liquid, it is impossible to completely remove a large amount of the coolant from the surface. Consequently, drips of liquid or a liquid membrane remain on the thin film. As a result, an irregularity of stretching easily occurs.
Further, in the cylindrical portion of the thin film that is formed by the part thereof which is in a melted state, the shape of the thin film is maintained by injecting compressed air thereinto. However, it is difficult to make a nozzle for injecting compressed air with a slit-like shape continuous in the circumferential direction. Therefore, there are defects in the method, as for example, the internal pressure applied to the thin film by means of streams of compressed air which are injected from the injecting nozzle become non-uniform. Or there are the other defects in the method such as the compressed air forming a thin layer even if the injecting nozzle can be made to be a slit-like shape. Also, intermittent fluctuations of the internal pressure lead to thickness deviations in the thin film, thereby causing thickness deviations of the film after the thin film is stretched.